Tracking Light-Induced Electron Transfer Towards O 2 in a Hybrid Photoredox-Laccase System

Photobiocatalysis is an interesting approach to use light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activity. Laccase, a robust multicopper oxidase, can be envisioned as a tool to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)3]2+ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires using an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions (O2●–) are scavenged by laccase leading to formation of water instead of H2O2. The net result is the photo accumulation, in an essentially irreversible way, of highly oxidizing [Ru(bpy)3]3+. This study provides a global scheme for the future use of laccase, in tandem with a light-driven oxidative process, using O2 as both a one-electron transfer relay and a 4-electron substrate to become the sustainable final electron acceptor in a such a hybrid photocatalytic process.